The present invention relates to a digital control circuit which controls the drive of a load on the basis of clock pulses. More particularly, it relates to a combustion control circuit for controlling the combustion of a burner in a water heater, an air heater or the like.
Combustion control circuits of the specified type are described in, for example, U.S. Pat. No. 4,145,179 and International Patent Application Laid-open No. WO 80/01604 (International Application No. PCT/JP80/00008, U.S. patent application Ser. No. 224,289).
The arrangement and operations of a combustion control circuit of this type will be explained with reference to FIGS. 1 and 2. Numeral 1 designates an integrated circuit device for combustion control, which is, e.g., `HA16605W` produced by Hitachi Ltd. The integrated circuit device 1 is provided with power supply input terminals 2a and 2b, an input terminal 3 for a clock pulse, an input terminal 4 for a flame detection signal, an input terminal 5 for a temperature detection signal, an output terminal 6 for driving a blower, an output terminal 7 for driving an ignition circuit, an output terminal 8 for driving a fuel valve, and an output terminal 9 for driving a safety device. It includes therein a timer 11 which keeps time by counting the clock pulses of the input terminal 3, and a control circuit 12 which supplies the output terminals 6, 7, 8 and 9 with driving signals at their respective timings on the basis of the signals of the input terminals 4 and 5 and signals from the timer 11.
Numeral 13 designates an A.C. source, numeral 14 a D.C. source circuit, numeral 15 an operation switch, numeral 16 a diode, numerals 17 and 18 voltage dividing resistors, and numeral 19 a noise preventing capacitor.
The operations of such arrangement will now be explained with reference to FIG. 2. Numerals at the left end of FIG. 2 correspond to those in FIG. 1. When the operation switch 15 is turned "on" and the input terminal 5 receives a signal indicating that the temperature of a load is lower than a preset value (at time t.sub.0), the control circuit 12 causes the timer 11 to start keeping time. The input terminal 3 is supplied with clock pulses obtained through the half-wave rectification of the alternating current of the A.C. source 13. The timer 11 begins to count the clock pulses from the input terminal 3. Thus, a prepurge period is started. At this time, the control circuit 12 delivers a driving signal to the output terminal 6 so as to start a prepurge operation.
When the timer 11 has counted a predetermined number of clock pulses, it supplies the control circuit 12 with an output, in response to which the control circuit 12 delivers driving signals to the output terminals 7 and8 so as to start an ignition operation (at time t.sub.1). Meanwhile, the timer 11 continues the counting operation. A safety period begins at the time t.sub.1. When a signal indicating the existence of a flame has been receives at the input terminal 4 (at time t.sub.2) within the safety period (t.sub.1 -t.sub.4), the control circuit 12 stops the driving signal of the output terminal 7 so as to establish a normal combustion state. If the flame detection signal 4 disappears, that is, if the flame is extinguished in the normal combustion state, the combustion is restarted from the prepurge operation by way of example. Further, if the load temperature exceeds a predetermined value in the normal combustion state so that the temperature detection signal 5 disappears, the control circuit 12 stops the combustion.
If the flame detection signal is not received within the safety period, the operation is repeated from the beginning of the prepurge operation. Alternatively, as indicated by broken lines in FIG. 2, the control circuit 12 stops delivering the driving signals of the output terminals 6, 7 and 8 and delivers an alarm signal from the output terminal 9 at the time t.sub.4, to stop the combustion operation.
When, as illustrated at (8), (4), (9) and (3) in the lower part of FIG. 2, the input terminal 3 is grounded or electrically disconnected accidentally or intentionally at the time t.sub.3 within the safety period, no clock pulse is received, and hence, the safety period is prolonged infinitely. This is very dangerous because fresh fuel may continue to be released in case of misfire (assuming that the D.C. source is kept energized).
The situation is similar in a case where the input terminal 3 is pulled up due to its contact with any other higher voltage part and where the ignition is not effected.
Regarding a circuit wherein the reception of an external signal, the delivery of an external signal or the exchange of an internal signal is performed in synchronism with a clock pulse, the interruption of the reception of the clock pulse causes a problem because the circuit becomes incapable of operating. For example, in a circuit wherein the signal of the input terminal 4 is received in synchronism with the clock pulse, even when a signal indicating the extinguishing of a flame has been received, the control circuit 12 cannot supply the output terminal 8 with a stop signal for the fuel valve without receipt of the clock pulse, so that the release of fuel is continued.
As a measure against this drawback, a second timer has been installed separately from the integrated circuit device 1. It is also designed to furnish the integrated circuit device 1 with two input terminals for the clock pulses. In either case, however, the cost is high.
On the other hand, U.S. patent application Ser. No. 458,454 has proposed an expedient according to which a clock pulse having a predetermined bias voltage is impressed on the clock pulse input terminal of an integrated circuit device, a circuit for detecting the presence or absence of the bias voltage is disposed in the integrated circuit device, and the operation is stopped in the absence of the bias voltage.